The present invention relates generally to ESC monitoring devices and ESC protective devices, and more particularly is a method of monitoring tribolelectric current generated in an operator""s body, and thereby controlling ESC levels on the operator""s body.
The present invention is based on the underlying mechanisms which cause electrostatic potentials to build up on human or other bodies. These mechanisms are used to provide improved and simplified methods for elimination of electrostatic potentials. The present invention also addresses the voltage potentials which are capacitively coupled into human or other bodies by power distribution wiring or by other naturally occurring or man-made electric fields.
The build up of electrostatic potentials on the human body when a person walks over a carpeted surface and when that person subsequently feels the unpleasant discharge when touching a door-knob has been experienced by countless numbers of people. Likewise, existence of capacitively coupled low frequency potentials, mostly from power distribution wiring, can be relatively easily demonstrated by touching an input of an audio amplifier with a finger, which results in a loud hum emanating from the speakers.
The capacitively coupled voltage potentials from power distribution wiring have been traditionally largely ignored because their peak amplitudes are relatively small in comparison with the ones from electrostatic origin. In practice, when measuring these potentials, one finds peak amplitudes well below one kilovolt, whereas peak amplitudes from triboelectric charging are easily an order of magnitude higher. The neglect is further compounded by the fact that a human body discharge below 3 kilovolts is usually not physically felt. However, state of the art electronic devices and micro-structures have become so electrostatic discharge sensitive that one can no longer neglect the effects of low voltage potential discharges such as the ones from capacitively coupled low power line electric fields. All further references to human body voltage potentials in the application therefore imply potentials of triboelectric origin as well as potentials resulting from capactively coupled electric fields.
In order to reduce and eliminate the detrimental effects of voltage potential discharges from human bodies, a number of devices have been developed. The most common of these devices is the xe2x80x9cgrounded wrist strapxe2x80x9d. The grounded wrist strap is basically a wrist-worn bracelet made of a conductive material. The bracelet makes electrical contact with a person""s skin, and is connected to protective earth or other electrical common reference plane (subsequently referred to as the xe2x80x9cgroundxe2x80x9d) via a lead wire. Usually a resistor with a value of at least one megaohm is inserted in this path-to-ground. The choice of the resistance value is a compromise between the need to adequately bleed-off the voltage potential charges, and, the need to avoid electric shock and/or electrocution when a person wearing such a grounded bracelet accidentally touches a live power distribution wire.
However, as shall be demonstrated below, the 1 Mohm or higher resistor does not reduce the voltage potentials sufficiently close to zero, and does thus not provide adequate protection during handling of state of the art electronic structures and devices. Under certain circumstances, instantaneous peak voltages of tens to hundreds of volts can be measured in spite of such grounding. State of the art Magneto Resistive (GMR, TMR, etc. . . . ) magnetic disk drive heads and sub-micron semiconductor structures can be destroyed with a human body voltage discharge of as low as 5 volts.
In addition to the wrist straps, it is also common industry practice to use xe2x80x9cWorkstation Monitorsxe2x80x9d in conjunction with the wrist straps. The purpose of the workstation monitors is at least twofold: (1) to establish a controlled path-to-ground, and, (2) to verify whether or not a wrist strap is being properly worn. Verification of whether or not a wrist strap is being properly worn is usually accomplished by dividing the wrist strap into two sections isolated from each other. In a typical detection approach, a small measuring current is sent through the wearer""s skin between the isolated sections. By measuring the voltage drop across the isolated sections, a decision can be made whether the user""s wrist is present or not. If a wrist is indeed present, it is further assumed that a proper connection to ground most likely exists.
The disadvantages of existing wrist straps, whether the split or the non-split version, in conjunction with their existing workstation monitors, are as follows:
1. The 1 Mohm or higher resistor in the path-to-ground allows for body voltage potential excursions well beyond the safe limits of state of the art electronic structures and devices.
2. The value of the resistor in the path-to-ground cannot be lowered due to safety considerations.
3. None of the existing wrist strap/workstation monitor combinations determine with scientific certainty whether a true path from wrist to ground actually exists. All known approaches rely on the assumption that the hardware components used in this path seldom fail, and, therefore, the path is assumed to be of high integrity.
4. The interface between commonly used wrist strap materials and the human skin is not optimum. Well understood bioelectric effects cause variability in the contact resistance. Bioelectric potentials, because of the ionic nature of the skin/wrist strap interface, do interfere with the small measurement currents used in the split wrist strap approaches. As will be demonstrated later, it is unlikely that existing wrist straps will ever allow reliable and repeatable paths-to-ground of sufficiently low resistance, so as to meet the necessary 5 volt and lower sensitivities.
5. Certain split wrist strap measurement circuits in existing workstation monitors apply voltages above 5 volts to the wrist-under-test, thus charging the body capacitance to these levels, hence possibly contributing to device destruction rather then to their protection.
The present invention intends to improve on or eliminate all these constraints and limitations. The present invention establishes a method of monitoring and controlling electrostatic charge on a human body. The method utilizes the discovery that the first order phenomenon in the charging of a human body to a voltage potential is in fact an electrical current, and that this current is driven from a near perfect current source of atomic nature. Proof of existence of this triboelectric current can be easily reproduced and demonstrated by connecting a human-under-test to a Current Amplifier, such as the Keithly 428 or equivalent, or, to a simple homemade current-to-voltage operational amplifier circuit with a 1 microamp per volt conversion gain. The output of the amplifier can be observed with a suitable recorder or oscilloscope while the human-under-test walks over a typical electrifying floor surface. In order to eliminate interference from often overwhelming capacitively coupled stray electric fields, the test is best done in a shielded room. Peak triboelectric currents generated by typical shoe sole/floor surface interactions were found to be as high as tens of microamperes. However, the vast majority, estimated at better then 95%, do not exceed 10 microamperes.
The voltage potential which develops on a human body, and which is measurable with any suitable electrometer or electrostatic voltmeter, is thus a direct result of a triboelectric current charging the body capacitance to an instantaneous voltage according to the general equation dv=(i*dt)/C. The typical body capacitance is 100 to 150 picofarads, and depends on the size, shape and posture of the body. In fact, the body voltage potential will continuously vary because of the continuous body shape or posture variations, all of which will change the body capacitance. This variation of body voltage potential is a secondary order effect in accordance with the law of conservation of charge (C1*V1=C2*V2=Q) which only can take place after the body capacitance was initially charged with a triboelectric current.
For purposes of electrical circuit analysis, the charging phenomenon can be modelled with an equivalent circuit comprising the triboelectric current source i(t), the body capacitance C, and the resistive path-to-ground R is shown in FIG. 1. R represents the parallel circuit of all intentional (such as the 1 Mohm or higher bleeder resistor) and unintentional (such as conductive and ionic leakage paths) resistive paths between the body and ground. From a circuit analysis or spice simulation perspective, it is a first-order circuit driven by a non-constant current source. The source is non-constant because the interactions between the shoe sole and the floor surface are erratic and unpredictable. The total response of this circuit, which yields the instantaneous body voltage potential v(t), invariably starts off with a part which is xe2x80x9ctransientxe2x80x9d in nature (a person starting to walk, for example, or having occasional and infrequent shoe-to-surface interactions for example) and may even out to a xe2x80x9csteady statexe2x80x9d type response when, for example, a person walks at a steady and regular pace for some distance. Mathematical circuit analysis and/or Spice simulation will show that the peak amplitude of v(t) under the xe2x80x9ctransientxe2x80x9d regime can be significantly higher than the one found under the xe2x80x9csteady statexe2x80x9d regime. This can be easily verified experimentally by measuring the body voltage potentials with a suitable electrometer or electrostatic voltmeter. The peak voltages measured during a person""s initial steps will be considerably higher than the ones measured during the balance of the walk. By the same token, and operator sitting at a workstation, and shuffling his/her feet occasionally and infrequently will produce considerably higher body voltage excursions that the same operator shuffling his/her feet continuously.
A typical current waveform associated with a single step with a nonconductive shoe on a non-conductive carpeted surface is shown in FIG. 2. The waveform can be broken down into four distinct segments: A) heel up, B) ball of the foot up, C) heel down, and, D) ball of the foot down. While the wave shape will generally approximate that shown in FIG. 2, the polarity of the waveform will depend on the specific materials present in a given shoe-to-surface interaction. The same shoe tracked in FIG. 2 would produce a waveform with reversed polarity if the floor surface is different, for instance a hardwood laminate.
When a person is grounded through a resistor, as in existing wrist strap protective devices, triboelectric current i(t) is forced through the parallel circuit formed by the grounding resistor R and body capacitance C. The waveform and the peak amplitude of the resulting voltage (v(t)) across the parallel circuit can be determined via mathematical and/or experimental methods. The peak amplitude will be proportional to both the peak amplitude of i(t) and the speed with which i(t) varies over time (di/dt). Experimentally, it has been determined that peak current amplitudes of tens of microamps and rates of rise as high as 5 milliamps/sec are not uncommon. When a current transient of this nature is forced to flow through a parallel combination comprising a 1 Mohm grounding resistor and a 100 picofarad body capacitance, a peak voltage potential v(t) of tens of volts will result Similarly, if the resistance in the path-to-ground is 10 Mohm, as in certain existing wrist strap and workstation monitor combinations, a peak body voltage potential of over one hundred volts will result. In both cases, these amplitudes are clearly beyond the survival limits of state of the art electronic structures and devices.
As mentioned above, it was determined experimentally that the majority of shoe-to-floor-surface interactions produce triboelectric peak currents not exceeding 10 microamperes, with rise times not exceeding 2.5 milliamps/second. From this basis, one can determine via mathematical and/or experimental methods, that for a given body capacitance of 100 picofarads, one can limit the peak body voltage excursions to 1 volt if the resistor value in the path-to-ground does not exceed 100 kiloohm. For all practical purposes, with path-to-ground resistances below a few hundred kiloohm, one can approximate the peak body voltage excursion by simple multiplication of the peak current times the path-to-ground resistance. The error made by omitting the effect of the body capacitance is minimal. The 100 kiloohm resistor value of course includes the contact resistance between the wrist strap and the skin of its wearer. As mentioned above, the resistance of current art wrist straps is highly erratic and unpredictable. In fact, values of hundreds of kiloohms on persons with dry skin, particularly when compounded by motion artifact, are quite common. It is thus highly unlikely that the existing wrist straps in combination with their existing workstation monitors will allow for the safe handling of very sensitive state-of-the-art electronic devices and structures which have damage thresholds of 5 volts or less.
Accordingly, it is an object of the present invention to provide a protection/monitoring system that limits potential body voltage excursions within the safe limits of state of the art electronic structures and devices.
It is a further object of the present invention to provide a methodology that allows the resistance in the path-to-ground to be safely lowered.
It is a still further object of the present invention to provide a means to determine with scientific certainty whether a true path from wrist to ground actually exists.
It is another object of the present invention to provide an improved interface between wrist strap materials and the human skin.
It is still another object of the present invention to reduce or eliminate voltages applied to the wrist-under-test, thereby reducing the body voltage potential.
The present invention is a method of monitoring ESC on a human body comprising the following steps:
a) Connecting the human body to a xe2x80x9cQuasi Virtual Groundxe2x80x9d. The maximum resistance in the path-to-ground does not exceed 10 Kohm, thus insuring that the majority of peak body voltage potential excursions will not exceed 100 millivolts. For all practical purposes, it comes down to the triboelectric current being shunted directly to ground.
b) Providing contact with the human skin via one or more electrodes. The preferred type of electrode is a silver/silver chloride electrode. The chemical, mechanical and electrical properties of silver/silver-chloride electrodes in contact with human skin have been studied extensively by the medical industry and are thus well optimized and understood. With the addition of an adequate gel between the electrodes and the skin, the contact resistance is typically below 2 Kohm, and, the resistance is stable over reasonable amounts of time (days), and is virtually not prone to motion artifact. The silver/silver chloride electrodes could be part of a designed for the purpose bracelet, or they could be pre-gelled disposable adhesive patches similar to the ones used for medical electrocardiography (Holter electrodes). Whatever the means of making contact with the skin, it is very important for the contact resistance to be as low as practical. As a fall back method, one can also use industry standard wrist straps, either the split or the non-split versions, with the herein described workstation monitor, but because of their inherent high and unstable skin contact resistance, body voltage potential excursions exceeding the 5 volt threshold limit are to be expected. Application of a topical conduction improving gel on the wrist could alleviate some of the problems. For the sake of simplification, the means of contact with the skin will be further referred to as the xe2x80x9cbody electrodexe2x80x9d.
c) Continuously monitoring the triboelectric current i(t) flowing into the xe2x80x9cQuasi Virtual Groundxe2x80x9d terminal. The amplitude and polarity of the triboelectric current can be made visible on any suitable visual display device such as, for example, a recorder, an oscilloscope, or a bar graph display with a linear, logarithmic, or custom scale.
d) Determining whether or not a person is properly wearing his/her body electrode and whether or not a true path from wrist to ground actually exists. This determination can be accomplished through a number of methods, none of them requiring a measuring current to be sent through a split body electrode. The methods will be described in detail below.
e) Including circuitry that immediately interrupts the path-to-ground when a dangerously large current is detected. In the present arrangement, with the path-to-ground being of relatively low resistive value, a danger for electroshock and/or electrocution exists in case of accidental contact with a live power distribution wire. Circuitry is therefore included in this novel workstation monitor which will detect and interrupt the path-to-ground nearly instantaneously. A suitable type of visual and audible alarm will be activated concurrently. Deliberate manual reset will be required in order to reestablish the path-to-ground.
f) Including circuitry that immediately interrupts the path-to-ground when the power supply to the workstation monitor is interrupted, either intentionally or accidentally. The path-to-ground will remain open until the deliberate manual reset has been executed.
An advantage of the present invention is that it limits potential body voltage excursions within the safe limits of state of the art electronic structures and devices.
Another advantage of the present invention is that it provides a means to determine with scientific certainty whether a true path from wrist to ground actually exists.
These and other objects and advantages of the present invention will become apparent to those skilled in the art in view of the description of the best presently known mode of carrying out the invention as described herein and as illustrated in the drawings.